1. Technical Field
The present disclosure relates to a terminal, a base station, a transmission method, and a reception method.
2. Description of the Related Art
3GPP LTE (3rd Generation Partnership Project Long Term Evolution) employs OFDMA (Orthogonal Frequency Division Multiple Access) as a communication technique for the downlink.
In wireless communication systems that employ 3GPP LTE, a base station (hereinafter, also referred to as “eNB”) sends a synchronization signal (SCH: Synchronization Channel) and a broadcast signal (PBCH: Physical Broadcast Channel) by using predetermined communication resources. In addition, a terminal (hereinafter also referred to as “UE (User Equipment)”) catches the SCH first so as to ensure synchronization with the base station. Subsequently, the terminal reads BCH information and acquires base-station specific parameters (e.g., the frequency bandwidth) (refer to, for example, 3GPP TS 36.211 V11.5.0, “Physical channels and modulation (Release 11),” December 2013, 3GPP TS 36.212 V11.4.0, “Multiplexing and channel coding (Release 11),” December 2013, and 3GPP TS 36.213 V11.5.0, “Physical layer procedures (Release11),” December 2013).
In addition, after acquiring the base-station specific parameters, the terminal sends a connection request to the base station and, thus, establishes connection with the base station. The base station sends control information to the terminal with which connection has been established via a control channel, such as PDCCH (Physical Downlink Control Channel), as needed. Thereafter, the terminal attempts to decode the control information contained in a received PDCCH signal (blind detection). That is, the control information includes a CRC (Cyclic Redundancy Check) portion masked with the terminal ID of a destination terminal in the base station. Accordingly, the terminal demasks the CRC portion of the received control information with the terminal ID of the terminal to determine whether the control information is destined for the terminal itself. If, as a result of the demasking in the blind detection, calculation of the CRC has no error, the terminal determines that the control information is destined for itself.
In addition, in LTE, HARQ (Hybrid Automatic Repeat Request) is applied to downlink data from a base station to a terminal. That is, the terminal feeds back, to the base station, a response signal indicating the result of detection of an error in the downlink data. The terminal performs CRC on the downlink data. If there is no error in the result of CRC calculation, the terminal feeds back acknowledgement (ACK) to the base station. However, if there is an error in the result of CRC calculation, the terminal feeds back negative acknowledgement (NACK) to the base station. To feed back the response signal (i.e., the ACK/NACK signal), an uplink control channel, such as a PUCCH (Physical Uplink Control Channel), is used.
Note that the above-described control information sent from the base station includes resource allocation information for identifying a resource allocated to the terminal by the base station. To send the control information, the PDCCH is used. The PDCCH is formed from at least one L1/L2 CCH (L1/L2 Control Channel). Each of the L1/L2 CCHs is formed from at least one CCE (Control Channel Element). That is, a CCE is a basic unit of mapping the control information to the PDCCH. In addition, when an L1/L2 CCH is formed from a plurality of CCEs, a plurality of CCE that are sequentially arranged are allocated to the L1/L2 CCH. In accordance with the number of CCEs needed for sending the control information to a terminal to which resources are to be allocated, the base station allocates the L1/L2 CCH to the terminal. Thereafter, the base station maps the control information to a physical resource corresponding to the CCE of the L1/L2 CCH and transmits the control information.
In addition, the CCEs are associated one-to-one with the resources that constitute the PUCCH (hereinafter, the resources are referred to as “PUCCH resources”). Accordingly, upon receiving the L1/L2 CCH, the terminal identifies a PUCCH resource corresponding to a CCE that constitutes the L1/L2 CCH and sends the ACK/NACK signal to the base station by using the PUCCH resource. However, if the L1/L2 CCH occupies a plurality of consecutive CCEs, the terminal sends the ACK/NACK signal to the base station by using one of the PUCCH resources each corresponding to one of the CCEs (e.g., the PUCCH resource corresponding to the CCE having the smallest index).
In addition, as illustrated in FIG. 1, the timing at which the terminal sends the ACK/NACK signal by using the PUCCH is in a subframe (a subframe n+K in FIG. 1) that is K subframes later than the subframe (a subframe n in FIG. 1) that has received the PDCCH signal and a PDSCH (Physical Downlink Shared Channel) signal having data allocated thereto by the PDCCH signal. For example, K=4 in FDD (Frequency Division Duplex).
As illustrated in FIG. 2, a plurality of ACK/NACK signals transmitted from a plurality of terminals are spread by the ZAC (Zero Auto-correlation) sequence having the Zero Auto-correlation characteristic in the time domain (by multiplying the ACK/NACK signal by the ZAC sequence) and are code-multiplexed in the PUCCH. In FIG. 2, (W(0), W(1), W(2), W(3)) represents the Walsh sequence having a sequence length of 4, and (F(0), F(1), F(2)) represents the DFT sequence having a sequence length of 3.
As illustrated in FIG. 2, in the terminal, the ACK/NACK signal is primarily spread by the ZAC sequence (the sequence length=12) in the frequency domain into a frequency component corresponding to a 1SC-FDMA (Single-Carrier Frequency Division Multiple Access) symbol first. That is, the ZAC sequence having a sequence length of 12 is multiplied by the components of the ACK/NACK signal in the form of a complex number. Subsequently, the ACK/NACK signal subjected to the primary spread and the ZAC sequence serving as a reference signal are secondarily spread by the Walsh sequence (the sequence length=4: W(0) to W(3)) and the DFT sequence (the sequence length=3: F(0) to F(2)), respectively. That is, the components of a signal having a sequence length of 12 (the ACK/NACK signal subjected to the primary spreading or the ZAC sequence serving as a reference signal) are multiplied by the components of an orthogonal sequence (the Walsh sequence or the DFT sequence), respectively. In addition, the secondarily spread signal is transformed into a signal having a sequence length of 12 in the time domain by the Inverse Discrete Fourier Transform (IDFT or IFFT (Inverse Fast Fourier Transform)). Thereafter, a cyclic prefix is added to each of the signals after IFFT. In this manner, a signal for one slot formed from 7 SC-FDMA symbols are generated.
A PUCCH is disposed on each end of the system band along the frequency domain. In addition, the PUCCH resource is allocated to each of the terminals on a subframe basis. Furthermore, a subframe is formed from two slots, and PUCCH frequency hopping occurs between the first slot and the second slot (inter slot frequency hopping).
The ACK/NACK signals from different terminals are spread (multiplied) by using the ZAC sequences defined by different cyclic shift amounts (Cyclic Shift Indices) and orthogonal code sequences corresponding to different sequence numbers (OC indices: Orthogonal Cover Indices). The orthogonal code sequence is a pair of the Walsh sequence and the DFT sequence. Note that the orthogonal code sequence is also referred to as “Block-wise spreading code”. Accordingly, the base station can separate the plurality of code-multiplexed ACK/NACK signals by using despreading and correlation processing (refer to, for example, Seigo Nakao, Tomofumi Takata, Daichi Imamura, and Katsuhiko Hiramatsu, “Performance enhancement of E-UTRA uplink control channel in fast fading environments,” Proceeding of 2009 IEEE 69th Vehicular Technology Conference (VTC2009-Spring), April 2009). FIG. 3 illustrates a PUCCH resource defined by the sequence number (the OC index: 0 to 2) of an orthogonal code sequence and a cyclic shift index (0 to 11) of the ZAC sequence. If the Walsh sequence having a sequence length of 4 and the DFT sequence having a sequence length of 3 are employed, a maximum of 36 (=3*12) PUCCH resources can be defined in the same time-frequency resource. However, all the 36 PUCCH resource are not always made available. FIG. 3 illustrates an example in which 18 PUCCH resources (#0 to #17) are made available.
In recent years, as an infrastructure for supporting the future information society, M2M (Machine-to-Machine) communication has been expected to realize a service through autonomous communication among devices without decision by the users. A particular example of an M2M system is Smart Grid. The smart grid is an infrastructure system for efficiently supplying life line, such as electricity or gas. For example, the smart grid performs M2M communication between a smart meter installed in a home or a building and a central server so as to autonomously and efficiently control the demand balance of resources. Other examples of an application of the M2M communication system include a monitoring system for commodities management or remote medical care and a remote management of stock and billing in an automatic vending machine.
In particular, as an application of the M2M communication system, much attention has been focused on a cellular system covering a wide communication area. The 3GPP has been studying M2M based on a cellular network to standardize LTE and LTE-Advanced in the name of MTC (Machine Type Communication). In particular, 3GPP has been studying Coverage Enhancement that further expands the coverage area to support MTC communication devices, such as a smart meter, installed in a coverage hole of an existing communication area, such as a basement of a building (refer to, for example, 3GPP TR 36.888 V12.0.0, “Study on provision of low-cost Machine-Type Communications (MTC) User Equipments (UEs) based on LTE,” June 2013).
To further extend the coverage area, MTC coverage enhancement plans to provide “Repetition” that transmits the same signal a plurality of times. More specifically, the study of performing repetition transmission in each of channels, such as PDCCH, PDSCH, and PUCCH, has been conducted.